Nanostructures (i.e., material having sizes that are on the order of a few nanometers) exhibit properties that are intermediate between the properties exhibited by atoms and molecules, and the properties exhibited by bulk solids. These unique properties, or quantum mechanical effects, make nanostructures promising candidates for various optical, optoelectronic, and microelectronic applications.
Nanostructure-based devices are expected to offer improved performance over more conventional devices. For example, indirect band gap semiconductors (e.g., silicon (Si) and germanium (Ge)) typically exhibit extremely low radiative efficiency in bulk materials. However, quantum confinement of the carriers in nanostructures may increase the radiative efficiency and emission energies of these semiconductors. Direct band gap semiconductors (e.g., indium-arsenide (InAs) and indium-phosphide (InP)) are already commonly used in lasers because of their high radiative efficiencies in bulk materials. However, quantum confinement of the carriers in nanostructures may lower threshold current densities and allow for temperature independent energy emission, while also increasing emission energies. These properties may also be fine-tuned by controlling the size of the nanostructures. In addition, with the movement toward smaller, high-bandwidth, low-power interconnects, nanostructures are providing a basis for various optoelectronic applications. For example, nanostructures may be used in optical interconnects for integrated circuits, telecommunications equipment, electronic equipment, etc. Nanostructures may also be used for biological sensors (e.g., capable of connecting with molecules in the human body), and as field emission electron sources (e.g., for flat panel displays), among other applications.
However, before any of these applications can be effectively realized, nanostructures of known size and having a narrow size distribution must be reliably produced. Current methods of fabricating nanostructures are unreliable, producing nanostructures having inconsistent or undesirable properties, and/or are expensive. For example, the Stranski-Krastanov method may be used to produce a coherent strained layer of three-dimensional nanostructure islands. However, the nanostructures are unstable and have varying optical properties. Nanofabrication using lithography and etching to form Si and Ge nanostructures is expensive, and the resulting nanostructures have poor optical properties. Other methods such as laser-assisted catalytic growth of “freestanding” nanowires, and colloidal chemical synthesis both produce nanostructures which are not embedded in a semiconductormaterial, making these nanostructures less desirable for device applications. Yet other methods for producing nanostructures are also known, such as anodizing and etching to form porous Si containing Si quantum wires, and ion implantation and annealing to form Si or Ge nanocrystals, for example, embedded in a SiO2 matrix.
A need remains for a relatively inexpensive and reproducible method of producing high-quality nanostructures. Additional advantages would be realized if the process were spontaneous, thereby reducing or altogether eliminating manual intervention. Still other advantages would be realized if the method allowed greater control over the growth process, and hence the properties of the resulting nanostructures. Other optical, optoelectronic, and microelectronic applications would also be possible if the nanostructures could be produced from a wide range of materials.